Enhancing the tonal and color characteristics of digital images using expansive and compressive tone scale functions

[0001] Reference is made to commonly-assigned U.S. patent application Ser. No. 10/178,260, filed Jun. 24, 2002, entitled “Enhancing the Tonal Characteristics of Digital Images” by Edward B. Gindele, U.S. patent application Ser. No. 10/246,856 filed Sep. 19, 2002, entitled “Enhancing the Tonal Characteristics of Digital Images Using Inflection Points in a Tone Scale Function” by Edward B. Gindele, and U.S. patent application Ser. No. 10/263,110 filed concurrently herewith, entitled “Enhancing the Tonal Characteristics of Digital Images Using Expansive and Compressive Tone Scale Functions” by Edward B. Gindele et al, the disclosures of which are incorporated herein by reference.

[0002] This invention relates to digital image processing and in particular to processing a digital image to enhance its color, brightness, and tone scale characteristics.

[0003] Many digital imaging systems enhance the contrast and lightness characteristics of digital images through the application of a tone scale curve. For a generalized tone scale curve ƒ( ), the input pixel value x is transformed to an output pixel value ƒ(x). The shape of the tone scale curve determines the visual effect imparted to the processed digital image. Some tone scale curves applied to digital image are independent of the pixel values in the digital image to be processed. Such image independent tone scale curves are useful for establishing a photographic look to the processed digital images. While image independent tone scale curves can be used to enhance many digital images, digital images that are either too high or low in contrast can benefit from the application of a tone scale curve that is responsive to the distribution of pixel values in the digital image to be processed. For image dependent tone scale curves, the mathematical formula used to generate the function ƒ(x) determines the degree and nature of the image enhancement.

[0004] When a compressive tone scale function, i.e. the tone scale function designed to reduce the numerical range of pixel values, is applied directly to image pixel data, the spatial detail in the image can be diminished. In addition, if the compressive tone scale function is applied directly to a color digital image that is in a red-green-blue representation, the tone scale function can reduce the colorfulness (saturation) characteristics of the processed digital image. In general, better results can be achieved if the compressive tone scale function is applied directly to the luminance signal of a luminance-chrominance image representation. This procedure leaves the chrominance signals unchanged but can lead to processed images that have diminished spatial detail.

[0005] In the method disclosed by commonly-assigned Gallagher and Gindele in U.S. Pat. No. 6,167,165, the spatial detail of the processed image can be mostly restored. In this method, a tone scale function is applied directly to the luminance signal and the spatial frequency detail of the tone scale modified luminance signal is adjusted inversely proportional to the slope of the applied tone scale function. The resultant processed digital images have more spatial detail and appear more natural.

[0006] A spatial filter can be used to apply a tone scale function to a digital image in such a manner that the image spatial detail will be relatively unaffected. This is particularly useful for compressive tone scale functions designed to reduce the image dynamic range. Such spatial filtering methods have been disclosed in commonly-assigned U.S. Pat. Nos. 6,317,521 and 6,285,798. When applying compressive tone scale functions to the luminance signal with a spatial filter, the color characteristics of the processed digital images look more natural even though the chrominance signals are not modified.

[0007] Some of the color problems incurred with the application of tone scale functions to luminance-chrominance representation digital images can be solved using the method disclosed by Gallagher and Gindele in commonly-assigned U.S. Pat. No. 6,408,103. In this method the chrominance signals are modified as a function of the slope of the applied tone scale function. Modifying the chrominance signals in this manner tends to alleviate the unnatural color effects that would otherwise result.

[0008] Some images naturally have a low numerical range to the pixel values and can be enhanced by applying an expansive tone scale function designed to increase the numerical range of pixel values. Simple histogram stretching methods are well known in the art. Typically a histogram is calculated from the image pixel values and the minimum and maximum pixel values are obtained. A tone scale function is constructed using a linear transform equation that expands the pixel values to achieve a predetermined goal. For example the minimum pixel value is mapped to a lower pixel value while the maximum pixel value is mapped to a higher pixel value. Applying an expansive tone scale function to image pixel data when in a red-green-blue representation has the effect of increasing image contrast, spatial detail, and color saturation. In general, the increase in contrast and spatial detail is viewed as an improvement in image quality. However, the increase in color saturation can lead to super-saturated color that can appear unnaturally colorful. Applying an expansive tone scale function to the luminance signal of a luminance-chrominance representation has the effect of increasing image contrast and spatial detail, but decreasing the apparent color saturation even though the chrominance signals are mathematically unaltered.

[0009] The above mentioned methods are individually designed to either avoid or remedy specific problems with image spatial detail and color characteristics when for applying tone scale functions to digital images. However, these methods have been designed to work primarily for compressive tone scale functions.

[0010] It is an object of the present invention to provide an improved way for applying a tone scale function to a digital image to enhance the tonal and color characteristics of the processed digital image.

[0011] This object is achieved in a method of enhancing the tonal and chrominance characteristics of a digital image, comprising the steps of:

[0012] a) receiving a source digital image including a plurality of pixels wherein the source digital image is in a luminance-chrominance representation having a luminance digital image channel and at least two chrominance digital image channels;

[0013] b) producing a tone scale function that can be used to enhance the tonal characteristics of the source digital image;

[0014] c) classifying the tone scale function into one of two or more categories based on the effect the tone scale function will have on the numerical range of pixel values of a processed digital image wherein the classification categories include expansive and compressive;

[0015] d) using the tone scale function and the luminance digital image channel to produce an enhanced luminance digital image channel; and

[0016] e) amplifying the chrominance digital image channels to produce modified chrominance digital channels when the tone scale function has been classified as expansive.

[0017] By practicing the present invention, the color characteristics of the digital image are significantly enhanced when also applying a tone scale function.

[0018] The present invention facilitates using functions that are either compressive or expansive to improve image luminance and color contrast wherein the color contrast characteristics are amplified when the applied tone scale function has been classified as expansive and are not amplified when the applied tone scale function has been classified as compressive.

[0039] In the following description, a preferred embodiment of the present invention will be described as a software program. Those skilled in the art will readily recognize that the equivalent of such software may also be constructed in hardware. Because image processing algorithms and systems are well known, the present description will be directed in particular to algorithms and systems forming part of, or cooperating more directly with, the method in accordance with the present invention. Other aspects of such algorithms and systems, and hardware and/or software for producing and otherwise processing the image signals involved therewith, not specifically shown or described herein, may be selected from such systems, algorithms, components and elements thereof known in the art. Given the description as set forth in the following specification, all software implementation thereof as a computer program is conventional and within the ordinary skill in such arts.

[0040] Still further, as used herein, the computer program may be stored in a computer readable storage medium, which may comprise, for example: magnetic storage media such as a magnetic disk (such as a floppy disk) or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable bar code; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program.

[0041] A digital image is comprised of one or more digital image channels. Each digital image channel is comprised of a two-dimensional array of pixels. Each pixel value relates to the amount of light received by the imaging capture device corresponding to the geometrical domain of the pixel. For color imaging applications, a digital image will typically consist of red, green, and blue digital image channels. Other configurations are also practiced, e.g. cyan, magenta, and yellow digital image channels. For monochrome applications, the digital image consists of one digital image channel. Motion imaging applications can be thought of as a time sequence of digital images. Those skilled in the art will recognize that the present invention can be applied to, but is not limited to, a digital image channel for any of the above mentioned applications.

[0042] Although the present invention describes a digital image channel as a two-dimensional array of pixels values arranged by rows and columns, those skilled in the art will recognize that the present invention can be applied to mosaic (non-rectilinear) arrays with equal effect.

[0043] The present invention may be implemented in computer hardware. Referring to FIG. 1, the following description relates to a digital imaging system which includes an image capture device 10, a digital image processor 20, image output devices 30a and 30b, and a general control computer 40. The system may include a monitor device 50 such as a computer console or paper printer. The system may also include an input control device 60 for an operator such as a keyboard and or mouse pointer. Still further, as used herein, the present invention may be implemented as a computer program and may be stored in a computer memory device 70, i.e. a computer readable storage medium, which may comprise, for example: magnetic storage media such as a magnetic disk (such as a floppy disk) or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable bar code; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program. Before describing the present invention, it facilitates understanding to note that the present invention is preferably utilized on any well known computer system, such as a personal computer.

[0044] Multiple image capture devices 10a, 10b, and 10c are shown illustrating that the present invention can be used for digital images derived from a variety of imaging devices. For example, FIG. 1 can represent a digital photofinishing system where the image capture device 10a may be a film scanner device which produces digital images by scanning a conventional photographic image, e.g. color negative film or slide film transparencies. Similarly, image capture device 10b could be a digital camera. The digital image processor 20 provides the means for processing the digital images to produce pleasing looking images on an intended output device or media. Multiple image output devices 30a and 30b are shown illustrating that the present invention may be used in conjunction with a variety of output devices which may include a digital photographic printer and soft copy display. It should also be noted that the present invention can be implemented within the hardware and software of a digital camera such that the digital images produced by the digital camera have been processed with the present invention prior to being exported by the digital camera.

[0045] The digital image processor 20 depicted in FIG. 1 is illustrated in more detail in FIG. 2. The cascaded image processing modules shown in FIG. 2 represents a digital image processing path. An original digital image 101 is received from one of the image input devices shown in FIG. 1 and processed to produce a rendered digital image 103 that can be realized on an image output device. A RLSE (relative log scene exposure) conversion module 310 receives the original digital image 101 and generates an RLSE digital image that is prepared for balancing and the application of a tone scale function. A scene balance module 320 receives the RLSE digital image and performs a lightness and color balance adjustment resulting in a source digital image. A tone scale module 330 receives the source digital image, generates a tone scale function from the source digital image and applies the tone scale function to the source digital image resulting in an enhanced digital image. If the RLSE conversion module 310 and the scene balance module 320 are omitted from the image processing path of modules, the tone scale module 330 receives the original digital image 101 directly as the source digital image. A rendering module 340 processes the enhanced digital image such that the pixel values of the generated rendered digital image 103 have been prepared to yield a pleasing result when used in conjunction with an output image device such as a digital printer. As part of the processing performed by the rendering module 340, a rendering function R(x) is applied to the pixel data of the enhanced digital image to achieve the input to output pixel mapping required of the output image device. The configuration of processing modules shown in FIG. 2 is used for processing digital images that are either monochrome or color in nature. That is, the original digital image 101 includes one or more digital image channels of pixels wherein each digital image channel relates to a different color or pixels such as, an original digital image 101 that includes red, green, and blue representation or a luminance-chrominance representation.

[0046] The tone scale functions produced are designed to work best with digital images that are in a relative log scene exposure representation (RLSE). That is, the pixels of the original digital image 101 have a logarithmic relationship with respect to the original scene intensities of light from which the original digital image 101 is derived. For example, the image capture device 10a shown in FIG. 1 can represent a photographic film scanner that produces digital images that have a linear or logarithmic relationship with the amount of transmitted light projected through a photographic negative or positive film transparency sample. If the pixels values of the resultant original digital image 101 have a logarithmic relationship with the received light (i.e. the pixels are in a density representation), the original digital image 101 is considered to be in a relative log scene exposure representation. If the pixels of the original digital image 101 have a linear relationship with the received light, the original digital image 101 is considered to be in a linear exposure representation since, to within a reasonable approximation, the amount of light transmitted through the photographic film sample is linearly proportional to the amount of light received from the original photographed scene. A linear exposure representation digital image can be transformed into a relative log scene exposure representation by a linear-to-logarithmic transform implemented as function or as look-up-table (LUT) with the RLSE module 310 shown in FIG. 2.

[0047] The most common representation of digital images produced with digital cameras is a rendered representation, i.e. the digital image has been processed such that it will be yield a natural appearing image on an electronic display device. For most digital images produced with digital cameras the intended output image device is a CRT monitor device sometimes referred to as a gamma domain representation. Rendered digital images can also be transformed into a relative log scene exposure representation using a LUT transform. For digital images produced with digital cameras, the transform includes an inverse of the power law function associated with the intended electronic display device followed by an inverse rendering tone scale function related to the rendering tone scale function that the digital camera used to produce the original digital image 101. Alternatively, the method disclosed by McCarthy et al. in commonly-assigned U.S. Pat. No. 6,282,311 can be used to produce an RLSE representation from a rendered digital image.

[0048] While the best results are achieved with digital images that are in a relative log scene exposure representation, the present invention can be used to enhance the appearance of digital images that are in other representations such as linear and gamma domain representations described above. In particular, the present invention can be used to process digital images that are in a visual domain such as CIELAB (one of the color spaces defined by the International Commission on Illumination abbreviated as CIE).

[0049] The scene balance module 320 receives the RLSE digital image from the RLSE conversion module 310 and performs a lightness and color balance adjustment. The resulting processed digital image is called the source digital image since it is the input digital image to the tone scale module 330. The lightness adjustment to the pixel data accounts for exposure variability in the original digital image 101. The present invention can be used with any algorithm that performs a lightness adjustment, or luminance balance, of the input pixel data. In particular, the lightness balance procedure includes calculating one or more prediction features from the RLSE digital image. These predication features are combined using a linear equation of the prediction features into a single brightness balance value that relates to an estimate of the pixel value corresponding to a theoretical 18% gray scene reflector. While there are many such prediction features that are useful, the present invention uses a spatial activity measure as the most important prediction feature. An RLSE luminance-chrominance digital image is generated from the RLSE digital image. Next, a spatial activity filter is applied to the luminance pixel data of the RLSE luminance-chrominance digital image. The spatial activity filter calculates the minimum difference of adjacent pixel values within a 3 by 3 pixel region and assigns the minimum difference to the pixel. Pixels with minimum difference values that exceed a predetermined threshold are averaged to produce the first prediction feature. The second prediction feature is calculated by dividing the luminance pixels of the RLSE luminance-chrominance digital image into four strips and calculating the average the maximum pixel value from each of the four strips. Other prediction features that have proved to be useful are the average pixel value and the 90% cumulative histogram pixel value.

[0050] After the brightness balance value of the RLSE digital image has been calculated, the color cast of the RLSE digital image is calculated that can be used to remove an overall color cast. The principle causes of color casts are variation in the color of the source illumination and secondarily the color fidelity of the image sensor that recorded the original digital image. A color balance position is calculated for the RLSE digital image which represents the chrominance coordinates of a theoretical color-neutral scene reflector. The color balance position is calculated using a two dimensional Gaussian weighting surface applied to the chrominance pixel data of the RLSE luminance-chrominance digital image. Although the chrominance pixel data can be averaged in an unweighted manner, better results have been obtained using the two dimensional Gaussian weighting surface. This is principally due to the de-emphasis of highly colorful scene objects from the calculation. A balance look-up-table is calculated for each color of the RLSE digital image using the calculated color balance position and the brightness balance value. The balance look-up-table is applied to the RLSE digital image to produce the source digital image for the tone scale module 330. As a result of the application of the balance look-up-table, pixel values in the RLSE digital image that have a value equal to the predicted balance value are transformed to a value equal to a system defined reference gray point. Similarly, pixels with corresponding chrominance values that correspond to the color balance position will be transformed to a color neutral position.

[0051] It should be noted that it is also possible to combine the operation of the lightness balance procedure described above with the construction of the tone scale function described in later detail below. The lightness balance operation is equivalent to adjusting the reference gray point used in the construction of the tone scale function. If the lightness balance operation is combined with the construction of the tone scale function, the reference gray point is image dependent since the pixels of the digital image being processed are used to calculate the reference gray point.

[0052] The rendering module 340 accepts the enhanced digital image from the tone scale module 330 and prepares the pixel data for display on an output image device. The rendering module 340 performs a color transformation, a tonal rendering transformation, and an output encoding transformation. The pixel data must be transformed such that the pixel data representing different original scene colors are appropriate for the color spectral characteristics associated with the output image device. This can be accomplished in a multiple step procedure. First the RLSE representation pixel data of the enhanced digital image is transformed into a linear representation. Next a color matrix transform is applied to the linear representation pixel data. The color matrix can be a 3 by 3 element matrix wherein the elements of the matrix are determined by analyzing a collection of imaged color patch targets and measuring the resultant color produced with the intended output image device.

[0053] The other task includes the transformation of the pixel data from the linear or relative-log-scene -exposure representation to a rendered representation with the application of a rendering function R(x). In general, the dynamic range of the enhanced digital image is much larger than can be displayed on typical output image devices such as CRT monitors or photographic paper. Therefore, if the pixel data of the enhanced digital image were received by the output image device directly, much of the pixel data would be clipped in the extreme light and dark parts of the image with a substantial loss of spatial detail. The rendering function performs a graceful roll-off of the pixel data such that the processed pixel data when displayed will result in a gradual loss of spatial detail as pixel values approach the limits of the output image device.

[0054] The present invention can be used with many different mathematical forms for the rendering function such as a rendering function produced with the method disclosed in commonly-assigned U.S. Pat. No. 5,300,381 by Buhr et al. In general, optimal results are obtained using rendering functions that have a sigmoid shape, i.e., rendering functions having a zero or near slope for extreme input pixel values and having a maximum magnitude slope that is achieved for mid-tone input values. Some rendering functions can have a relatively high slope for the darkest domain of input pixel values but many rendering functions have a zero or near zero slope characteristic for the lightest domain of pixel values. The rendering function, to an extent, mimics the photo response characteristics of photographic paper used in analog imaging applications.

[0055] Although it is possible to use a rendering function that is dependent on the digital image being processed, the preferred embodiment of the present invention uses a sigmoid shaped rendering function that is independent of the digital image. Image dependent modification to the digital image being processed is performed by the tone scale module 330 prior to the application of the rendering function. Thus, the tone scale module 330 is responsible for scene dependent changes to the image content while the rendering module 340 is responsible for preparing the final processed image for an image output device.

[0056] FIG. 3a shows a graph of an example rendering function R(x) (indicated by curve 590) that is suitable for use with the present invention. Point 591 corresponds to an input pixel value equal to the reference gray point. Point 592 indicates the rendering function response for a highlight pixel value corresponding to a light region in the original digital image 101. Point 593 indicates the rendering function response for a shadow pixel value corresponding to a dark region in the original digital image 101. Point 595 indicates the rendering function response corresponding to brightest reproduced output value of the output image device. Similarly, point 596 indicates the rendering function response corresponding to darkest reproduced output value of the output image device. Point 597 indicates the point on the rendering function for which the instantaneous slope has a maximum magnitude which does not necessarily coincide with the reference gray point 591. The point of maximum magnitude slope is also the inflection point of the sigmoid function, i.e. a local maximum or minimum in the corresponding slope function of the rendering function R(x). The example rendering function shown in FIG. 3a is appropriate for a digital imaging system that uses a 12-bit pixel value representation for processing digital images. Many output display devices accept digital images with an 8-bit pixel value representation. The example rendering function shown in FIG. 3a maps 12-bit input pixel values ranging from 0 to 4095 to output rendering pixel values ranging from 255 to 0.

[0057] The example rendering function shown in FIG. 3a is typical for use with a digital printer that produces digital photographic prints. The output pixel value scale shown can relate to photographic paper densities, e.g. the digital printer can be calibrated such that each pixel code value is equal to 100 times the optical density that will be produced on the print. However, the present invention can also be used with electronic display devices. A typical rendering function for use with an electronic display device is shown in FIG. 3b as indicated by curve 598. Note that the rendering function R(x) shown in FIG. 3b produces higher numerical output rendering pixel values for higher numerical input pixel values. For a typical computer monitor device, the output pixel values will range from 0 (representing black) to 255 (representing white). The example rendering function shown in FIG. 3b maps 12-bit input pixel values ranging from 0 to 4095 to output rendering pixel values ranging from 0 to 255.

[0058] The last operation performed by the rendering module 340 is the encoding of the output pixel values for use with an output image device. Most output image devices are calibrated to accept pixel data with a known implied relationship. For example, some digital image printers are calibrated to accept visual lightness related pixel data while other devices are calibrated for optical density related pixel data. The encoding operation performs an intensity transformation that prepares the pixel data for the specific device.

[0059] The rendering module 340 can also perform the operation of preparing the image pixel data of the enhanced digital image for an unspecified output image device. For example, the pixel data can be transformed into CIE XYZ coordinates such as defined by the International Image Consortium's Profile Connection Space.

[0060] Referring to FIG. 2, the chain of image processing modules is designed to work well with or without the inclusion of the tone scale module 330. The scene balance module 320 accommodates exposure and illumination system variability. The rendering module 340 prepares the image data for viewing on an output image device. It will be appreciated by those skilled in the art that the digital image processor 20 as depicted in FIG. 2 without the use of the tone scale module 330 will yield acceptable photographic results. That is, the contrast, lightness, and color reproduction of the system are set for optimal photographic results for the majority of typical digital images. The inclusion of the tone scale module 330 enhances the appearance of the processed digital images such that digital images that deviate from the norm, in terms of dynamic range, will be tonally enhanced and digital images that are at the norm for the system will be unaltered by the tone scale module 330. For a typical implementation of the present invention, the dynamic range of the original digital images 101 (ratio of corresponding original scene intensities based on the extremes of the image) is approximately 64 to 1. The enhanced digital images corresponding to processed original digital images 101 that have approximately a 64 to 1 dynamic range will be little affected by the tone scale module 330. Conversely, the enhanced digital images corresponding to processed original digital images 101 that have a dynamic range greater than 64 to 1 and less than 64 to 1 can be significantly enhanced by the tone scale module 330, i.e. rendered with more detail and more color saturation. In particular, low contrast original digital image will, in general, experience a contrast gain, while high contrast original digital images will, in general, experience a reduction in contrast resulting in more spatial detail and more saturated color.

[0061] The tone scale module 330 depicted in FIG. 2 is illustrated in more detail in FIG. 4. A source digital image 102 is received by the analysis image generator 250, which produces a lower spatial resolution digital image from source digital image 102 called an analysis digital image 201. The tone scale function generator 230 receives the analysis digital image 201, analyzes the content of the analysis digital image 201, and produces the tone scale function 203. The tone scale function 203 is a single valued function that is defined for the range of pixel values in the source digital image 102. The tone scale function applicator 240 applies the tone scale function 203 to the source digital image 102 to generate the enhanced digital image 104. The shape of the tone scale function 203 determines the contrast and lightness changes that will be imparted to an enhanced digital image 104.

[0062] A tone scale function applicator 240 shown in FIG. 4 is shown in more detail in FIG. 5. A tone scale function classifier 241 receives and analyzes the tone scale function 203. The tone scale function 203 is classified into one of two or more categories based on the effect the tone scale function 203 will have on the numerical range of pixel values of a processed digital image. In a preferred embodiment, the tone scale function 203 is classified as a compressive function or an expansive function. The tone scale function 203 is classified as a compressive function if the range of output numerical pixel values is less than the range of input numerical pixel values. Conversely, the tone scale function 203 is classified as an expansive function if the range of output numerical pixel values is greater than the range of input numerical pixel values. The tone scale function classifier 241 also receives an input digital image 401. If the tone scale function 203 is classified as a compressive function, the input digital image 401 and the tone scale function 203 are directed to a compressive function applicator 242. Similarly, if the tone scale function 203 is classified as an expansive function, the input digital image 401 and the tone scale function 203 are directed to the expansive function applicator 242. Therefore, either the compressive function applicator 242 or a expansive function applicator 244 is used to process the input digital image 401 to produce an output digital image 409 which is the enhanced digital image 104 shown in FIG. 4. In particular, the compressive function applicator 242 uses the tone scale function 203, the input digital image 401, and a spatial filter to generate an output digital image 409 when the tone scale function has been classified as compressive, and the expansive function applicator 244 uses the tone scale function 203 and the input digital image 401 without a spatial filter to generate the output digital image 409 when the tone scale function 203 has been classified as expansive.

[0063] While the present invention can be used with any method of generating a tone scale function, the preferred embodiment uses tone scale functions that are constructed from two tone scale segments. However, regardless of the tone scale function construction method, the tone scale function is classified as either compressive or expansive. The present invention can be used with any tone scale function classification method. For example, the average slope of the tone scale function 203 can be calculated and compared with a pre-determined threshold value. If the average slope is greater than the pre-determined threshold the tone scale function 203 is classified as an expansive function. Otherwise the tone scale function 203 is classified as a compressive function.

[0064] The preferred embodiment of the present invention uses a histogram based method for determining the tone scale function classification. The input digital image 401 is analyzed by calculating a histogram function of the luminance pixel information, i.e. a frequency of occurrence function. The cumulative 1.0 and 99.0 fixed percentile luminance pixel values are calculated from the histogram function and denoted by P₁ and P₉₉ respectively. The value of the corresponding output pixel values produced with the tone scale function 203 are recorded as T₁ and T₉₉ respectively. The tone scale function 203 is classified as a compressive function if the difference between T₉₉ and T₁ is less than or equal to the difference between P₉₉ and P₁. Otherwise the tone scale function 203 is classified as an expansive function. Other classification rules can also be used to classify the tone scale function 203. For example, the tone scale function 203 can be classified as expansive only if the ratio of the difference between the T₉₉ and T₁ relative to the difference between the difference between P₉₉ and P₁ exceeds a predetermined threshold value.

[0065] The compressive function applicator 242 shown in FIG. 5 is shown in more detail in FIG. 6a. A LCC conversion module 210 receives the input digital image 401 in the form of a red-green-blue image representation and generates a luminance digital image 107 and a chrominance digital image 109. A pedestal generation module 410 receives the luminance digital image 107 and produces A pedestal digital image 403. The pedestal generation module 410 employs a spatial filter, applied to the luminance digital image 107, that removes texture from the luminance digital image 107 but leaves edge information. In the context of the present invention, texture refers to image content including fine spatial detail and spatial shading structure. As a natural consequence of the filter, noise is also removed. Thus the pedestal digital image 403 is a highly smoothed version of the luminance digital image 107 with much of the edge content of the luminance digital image 107. The pedestal digital image 403 is subtracted from the luminance digital image 107 by a difference module 420 resulting in a texture digital image 402. It will be appreciated that all of the information of the luminance digital image 107 is contained between the pedestal digital image 403 or the texture digital image 402. A tone scale modifier 430 applies the tone scale function 203 directly to the pixels of the pedestal digital image 403 resulting in a tone adjusted digital image 407. Therefore, the tonal characteristics of the pedestal digital image 403 are enhanced by the application of the tone scale function 203 without affecting the texture content of the luminance digital image 107. An addition module 440 combines the tone adjusted digital image 407 with the texture digital image 402 to produce an enhanced luminance digital image 113. Therefore, the enhanced luminance digital image 113 is a tone scale enhanced version of the luminance digital image 107 that has the texture image structure content of the luminance digital image 107. An RGB conversion module 220 receives the enhanced luminance digital image 113 and the chrominance digital image 109 and produces the output digital image 409.

[0066] Applying a tone scale function by using a spatial filter is particularly advantageous when the tone scale function is compressive. When compressive tone scale functions are applied directly to an input digital image, the tonal characteristics will be enhanced but the texture image structure can be diminished in magnitude. Using a spatial filter to apply the tone scale function can achieve the desired tone scale enhancement while maintaining the texture image structure. A variety of spatial filters can be used to achieve an improved result. The present invention uses the spatial filter disclosed in commonly-assigned U.S. Pat. No. 6,317,521 by Gallagher and Gindele which employs the use of a control, or masking, signal to preserve the edges in the pedestal digital image 403. In this method, the spatial filter produces a first signal containing predominantly low-pass spatial frequency information (the pedestal digital image 403) and second signal containing predominantly high-pass spatial frequency information (the texture digital image 402) from the luminance digital image channel such that when the first and second signals are added together the resultant signal would be equal to the luminance digital image channel. This separation of signals is accomplished with a non-linear operation that modifies the spatial filtering based on detecting edges within a low-pass frequency version of the luminance information. The tone scale function 203 is then applied to the first signal to produce a modified first signal. The modified first signal is then combined with the second signal to produce a modified luminance digital image channel.

[0067] The present invention can also be used with a similar method disclosed by Lee in commonly-assigned U.S. Pat. No. 6,285,798. Another spatial filter that can be employed is a simple low-pass filter, such as a two dimensional Gaussian filter. However, if a low-pass filter is used, the dimension of the Gaussian standard deviation parameter should be very large, e.g. one quarter of the dimension of the digital image being processed.

[0068] It should also be noted that the input digital image 401 input to the compressive function applicator 242 can be processed with the spatial filtering technique directly in a red-green-blue image representation. For this embodiment, the red, green, and blue color digital image channels of the input digital image 401 are spatially filtered individually using the spatial filter described above to produce a first and second signal for each digital image channel. The first and second signal for each digital image channel when added together result in a signal equal to the unfiltered digital image channel. The tone scale function 203 is then applied to the first signal corresponding to each digital image channel to produce a modified first signal. The modified first signal from each digital image channel is then combined with the corresponding second signal to produce a modified digital image channel of each color wherein the modified digital image channels constitute the enhanced digital image 104.

[0069] It is important to note that for tone scale functions classified as compressive functions the chrominance characteristics (the color characteristics apart from the luminance characteristics) of the enhanced digital image 104 are essentially the same as the source digital image 102. This is true for both the methods of applying the tone scale function 203 to a red-green-blue image representation and applying the tone scale function 203 to a luminance-chrominance image representation. In general, compressive tone scale functions are applied to enhance the luminance characteristics of a digital image. Therefore, the is no need to modify the chrominance characteristics of the processed digital images in a manner related to the nature of the applied compressive tone scale function. This should not be interpreted to imply that no further color enhancement to the enhanced digital image 104 is necessary or advantageous. However, no further processing of the chrominance characteristics are necessary to compensate for the applied compressive tone scale function.

[0070] The expansive function applicator 244 shown in FIG. 5 is shown in more detail in FIG. 6b. The LCC conversion module 210 receives the input digital image 401 in the form of a red-green-blue image representation and generates a luminance digital image 107 containing one digital image channel and a chrominance digital image 109 containing two digital image channels. The tone scale modifier 430 applies the tone scale function 203 directly to the pixels of the luminance digital image 107 resulting in the enhanced luminance digital image 113. Therefore, the tonal characteristics of the luminance digital image 107 are enhanced by the application of the tone scale function 203 while also amplifying the texture content of the luminance digital image 107. A chrominance modifier 432 receives the tone scale function 203, the luminance digital image 107, and the chrominance digital image 109 and amplifies the chrominance pixel values in accordance with the tone scale function 203. Specifically, the chrominance modifier 432 first calculates a corresponding slope function s( ) for the received tone scale function 203. Next a chrominance amplification factor s(α) is calculated for each luminance pixel value (λ) given expression (1)
α(λ)=μ(s(λ)−1.0)+1.0  (1)
where the parameter μ regulates the degree of chrominance amplification. For example, if the parameter μ is set to 1.0, the chrominance pixel values will be amplified by the same degree as the luminance pixel values. That is, the luminance contrast is increased by the same percentage as the color contrast. If the μ parameter is set to zero, the chrominance pixel values will remain the same and there will be no color contrast amplification. Through experimentation, the preferred value for the μ parameter has been determined to be 0.5. By setting the μ parameter to 0.5, low contrast images can be enhanced by an expansive tone scale function without excessively amplifying the color. This is particularly useful for enhancing low contrast images that contain faces.

[0071] For digital imaging applications that require or desire a high degree of chrominance amplification (approximately equivalent to using parameter μ=1.0 in Equation (1) above), an alternative embodiment of the expansive function applicator can be used. For this alternative embodiment, the LCC conversion module 210 and the RBG conversion module 220 shown in FIG. 6b can be omitted. The tone scale modifier 430 receives the input digital image 401 directly in a red-green-blue image representation and produces the output digital image 409. For this alternative embodiment of the expansive function applicator 244, the corresponding slope function does not need to be calculated. If the input digital image 401 is received in an LCC representation, the expansive function applicator 244 first converts the input digital image 401 into an RGB representation and then applies the tone scale function 203 directly to each digital image channel of the RGB representation of the input digital image 401. If the input digital image 401 is received in a red-green-blue representation, the expansive function applicator 244 applies the tone scale function 203 directly to each digital image channel of the input digital image 401 and the compressive function applicator 242 performs the image processing steps as described above.

[0072] In a still further alternative embodiment, the input digital image 401 is received in a multi-color image representation, such as a red-green-blue representation, and both the compressive function applicator 242 and the expansive function applicator 244 operate on the red-green-blue image pixel data. In this alternative embodiment, the expansive function applicator 244 applies the tone scale function 203 to the individual digital image channels of the multi-color representation input digital image 401. However, the compressive function applicator 242 uses the spatial filtering method described above to separate each of the digital image channels into two signals. That is, a first signal that includes mostly low-spatial frequency information and a second signal that includes mostly high-spatial frequency information. The tone scale function 203 is applied to the first signal part derived from each digital image channel to produce a modified first signal. The output digital image 409 is generated by combining the modified first signal with the second signal resulting in a modified multi-color representation digital image. The process of applying the tone scale function 203 to each channel of a multi-color representation digital image with a spatial filter preserves both the color contrast and spatial detail when the applied tone scale function is a compressive function. The process of applying the tone scale function 203 to each channel of a multi-color representation digital image without a spatial filter amplifies both the color contrast and spatial detail when the applied tone scale function is an expansive function.

[0073] Thus it will be appreciated that for tone scale functions that have been classified as expansive, the chrominance characteristics (also referred to as the color contrast) of the resultant enhanced digital image 104 are amplified relative to the source digital image 102. This is true for all the embodiments of the present invention described herein. For example, in the preferred embodiment, the tone scale function 203 is applied to the luminance digital image and the chrominance digital image channels are amplified directly when the tone scale function is classified as an expansive function. In the alternative embodiment, the tone scale function 203 is applied directly to a red-green-blue image representation digital image when the tones scale function is classified as an expansive function. In the latter case, the chrominance characteristics are amplified by virtue of the expansive nature of the tone scale function 203.

[0074] It will also be appreciated that the number of digital image channels of a digital image to be processed can vary based on the classification of the applied tone scale function. For example, when the tone scale function 203 has been classified as a compressive function, the tone scale function can be applied to just a luminance digital image channel. However, when the tone scale function 203 has been classified as an expansive function, the tone scale function 203 can be applied to multiple digital image channels of a multi-color representation digital image. Conversely, as described above, the tone scale function 203 can be applied to multiple digital image channels of a multi-color representation digital image using a spatial filter when the tone scale function 203 has been classified as a compressive function and only the luminance digital image channel tone scale function 203 has been classified as an expansive function.

[0075] The LCC module 210 shown in FIG. 6a and FIG. 6b employs a 3 by 3 element matrix transformation to convert the red, green, and blue pixel values of the source digital image 102 into luminance and chrominance pixel values. Let the variables R_ij, G_ij, and B_ij refer to the pixel values corresponding to the red, green, and blue digital image channels located at the i^th row and j^th column. Let the variables L_ij, GM_ij, and ILL_ij refer to the transformed luminance, first chrominance, and second chrominance pixel values respectively of an LCC representation digital image. The 3 by 3 elements of the matrix transformation are described by (2).
L_ij=0.333 R_ij+0.333 G_ij+0.333 B_ij  (2)
GM_ij=−0.25 R_ij+0.50 G_ij−0.25 B_ij
ILL_ij=−0.50 R_ij+0.50 B_ij

[0076] The RGB conversion module 220 shown in FIG. 6a and FIG. 6b employs a 3 by 3 element matrix transformation to convert the luminance and chrominance pixel values into red, green, and blue pixel values by performing the inverse matrix operation to the LCC conversion module 210. The matrix elements of the RGB conversion module are given by (3) and represents the inverse matrix of the matrix given by (1).
R_ij=L_ij−0.666 GM_ij−ILL_ij  (3)
G_ij=L_ij+1.333 GM_ij
B_ij=L_ij−0.666 GM_ij+ILL_ij

[0077] A tone scale function generator 230 shown in FIG. 4 is described in more detail hereinbelow. The tone scale function generator 230 can be used in either an automatic mode, wherein the tone scale function 203 is calculated using the pixels of the analysis digital image 201, or in a manual mode, wherein the tone scale function 203 is calculated using user input selections 231 provided via a graphical user interface.

[0078] The tone scale function 203, is a single valued function, i.e. one output pixel value for each input pixel value, defined for the range of pixels values in the source digital image 102. The shape of the tone scale function 203 is important since the shape determines the effect on the processed digital images. The tone scale function 203 is constructed from two function segments wherein the two function segments share just one input pixel value in common. A reference gray point pixel value corresponding to an 18% scene reflector is defined as the input pixel value in common that divides the function domain into the two tone scale segments. The function segment relating to the brighter image regions, i.e. image regions corresponding to bright regions of the original photographed scene, is called the highlight tone scale segment. The function segment relating to the darker image regions, i.e. image regions corresponding to dim regions of the original photographed scene, is called the shadow tone scale segment. It should be noted that the tone scale function 203 is a continuous function insofar as the implementation in computer software and/or hardware will allow. It should also be noted that the tone scale function 203 can have a continuous first derivative.

[0079] An important aspect of the construction of the tone scale function 203 is the use of a different mathematical function for two different tone scale segments. Using a different mathematical function for each tone scale segment allows the tone scale function 203 to be independently controllable in each of the tone scale segments allowing for a greater degree of customization for the digital image to be processed.

[0080] The pixel polarity of a digital image used in a digital imaging system is an arbitrary decision made by the system architect. For example, positive pixel polarity digital images have pixels wherein higher numerical values relate to more light having been received. Conversely, negative pixel polarity digital images have pixels wherein higher numerical values relate to less light having been received. The present invention can be used with digital images of either pixel polarity. However, in the interest of clarity, the following description will assume a positive pixel polarity convention. Those skilled in the art will recognize that references made to the increasing or decreasing function slope values are with respect to positive pixel polarity digital images. The description of function slope characteristics must be reversed for systems using negative pixel polarity digital images. This is an important aspect of interpretation since mathematically an increasing or decreasing function is defined with respect to numerically increasing abscissa values. For example, in the description of the construction of the highlight and shadow component functions given hereinbelow, some shadow component functions are described as having a monotonically increasing slope property while some highlight component functions are described as having a monotonically decreasing slope property. This description is with regard to a positive pixel polarity convention. For a negative pixel polarity convention, the equivalent shadow component functions would be described as having a monotonically decreasing slope property while the equivalent highlight component functions would be described as having a monotonically increasing slope property. Similarly, for a positive pixel polarity convention, the tone scale function has a slope function that is always greater than or equal to zero. Conversely, or a negative pixel polarity convention, the tone scale function can have a slope function that is always less than or equal to zero.

[0081] The highlight tone scale segment is constructed from one or more component functions some of which satisfy the following constraints: 1) the component function can have a monotonically decreasing slope function for all input pixel values equal to or greater than the reference gray point, and 2) the component function can have a monotonically increasing function value for all input pixel values equal to or greater than the reference gray point and less than or equal to a maximum input pixel value expressed in the digital image. At the reference gray point, the two tone scale function segments can have the same value. A function is monotonic over a given domain if the function has constant sign (for digital implementations the slope function is taken as a reasonable approximation of the first derivative function). It should also be noted that the function characteristics for input pixel values greater than what is expressed in a particular image is an academic issue since no pixels will be affected. Both of the above mentioned constraints are important and require some explanation.

[0082] The highlight tone scale segment relates to the bright pixels, i.e. pixels relating to more light having been received. In general, for high dynamic range digital images the corresponding rendered digital images produced without the present invention have little or no spatial detail in the very brightest image regions. This is a consequence of the overall high system contrast required to pleasingly render digital images of average dynamic range. Therefore, for high dynamic range digital images some of the image content contained in the bright pixels cannot be rendered such that spatial detail modulation is preserved in the rendered digital image 103. Improved spatial detail modulation can be achieved if the tone scale function 203 maps high input pixel values to lower output pixel values. This results in processed digital images with darker highlight content in the resultant rendered digital image 103. There are many functions that can perform such an input to output mapping operation. However, monotonically increasing functions have been experimentally determined to be more robust, i.e. produce fewer image artifacts, than functions that are not monotonic.

[0083] While many monotonic functions can achieve the operation of mapping high input pixel values to lower output pixel values, all functions impose some form of compromise in contrast with regard to image regions corresponding to different average pixel values. In particular, the instantaneous slope value (first derivative) of the component functions used to construct the highlight tone scale segment can significantly affect the perception of contrast and spatial detail modulation in the resultant rendered digital image 103. Therefore, the highlight tone scale segment constructed using component functions having a monotonically decreasing instantaneous slope value can improve the rendering of spatial detail modulation for bright image regions by mapping highlight pixels to lower output pixel values. Image regions corresponding to higher instantaneous slope values within the domain of the highlight tone scale segment tend to preserve more image detail modulation. Thus the monotonically decreasing instantaneous slope condition advantages image content corresponding to pixel values that are numerically closer in value to the reference gray point. In general, important image content, such as the main subject region, tends to be numerically closer to the reference gray point while background image content tends to be exhibited more uniformly with regard to pixel values.

[0084] Similarly, the shadow tone scale segment is constructed from one or more component functions some of which satisfy the following constraints: 1) the component function can have a monotonically increasing slope function for all input pixel values equal to or less than the reference gray point, and 2) the component function can have a monotonically increasing function value for all input pixel values equal to or less than the reference gray point and greater than or equal to a minimum input pixel value expressed in the digital image. Similarly, the monotonicity property of the component functions used to construct the shadow tone scale segment relates to more robust image quality results. The monotonically increasing slope function property of the component functions used to construct the shadow tone scale segment is similarly important since this condition also advantages image content corresponding to pixel values that are numerically closer in value to the reference gray point. For high dynamic range images, the monotonically increasing slope function property of the component functions used to construct the shadow tone scale segment achieves a low input pixel value to higher output pixel value mapping operation. This results in processed digital images with lighter shadow content in the resultant rendered digital image 103.

[0085] A natural consequence of the above mentioned slope function constraints produces tone scale functions that have high slope function values at the reference gray point. Therefore the choice of the reference gray point value is important since it determines which regions in images will experience high slope function values. A reference gray point value corresponding to an 18% gray scene reflector is chosen since it represents approximately the midpoint of perceptual lightness. Other choices for the value of the reference gray point can also produce excellent results. Reasonable values for the reference gray point range from a 10% scene reflector value to a 25% scene reflector value.

[0086] In a first embodiment of the tone scale function generator 230 shown in FIG. 4, either the highlight or the shadow tone scale segment is constructed using a single component function based on exponential functions. The component function used for the highlight tone scale segment is given by the formula (4)
ƒ_h1(x)=β_h1(1−e^{−(x−xρ)/αh1})+x_ρ  (4)
where x₉₂ represents the reference gray point, and β_h1 and α_h1 are numerical constants that determine the shape and slope of the component function ƒ_h1(x). The component function used for the shadow tone scale segment is given by the formula (5)
ƒ_s1(x)=β_s1(1−e^{−(x−xρ)/αs1})+x_ρ  (5)
where β_s1 and α_s1 are numerical constants that similarly determine the shape and slope of the component function ƒ_s1(x). If a slope constraint of 1.0 is imposed at the reference gray point, the constants β_h1 and β_s1 are equal to α_h1 and α_s1 respectively. For this condition, the equations for the functions ƒ_h1(x) and ƒ_s1(x) are given as (6) and (7)
ƒ_h1(x)=α_h1(1−e^{−(x−xρ)/αh1})+x_ρ  (6)
ƒ_s1(x)=α_s1(1−e^{−(x−xρ)/αs1})+x_ρ  (7)
and the expression for the tone scale function 203 T(x) is given by (8).
T(x)=ƒ_h1(x) for x>=x₉₂   (8)
T(x)=ƒ_s1(x) for x<x_ρ
The highlight component function is constrained to pass through a specified coordinate point defined by an abscissa value x_ho that results in an ordinate value x_w as given by (9).
x_w=α_h1(1−e^{−(xho−xρ)/αh1})+x_ρ  (9)
This constraint achieves a highlight white point mapping objective. For the highlight component function, the white point value x_w is predetermined based on the pixel value that is preferably mapped by the rendering function R(x) to correspond to a photographic paper density of approximately 0.2. With the variables x_w and x_ρ defined, the value of the variable α_h1 can be solved for a given value of x_ho using expression (9) by an iterative numerical solution. Similarly, the shadow component function is constrained to pass through a specified coordinate point defined by an abscissa value x_so that results in an ordinate value x_b as given by (10).
x_b=α_s1(1−e^{−(xso−xρ)/αs1})+x_ρ  (10)
This constraint achieves a shadow black point mapping objective. For the shadow tone scale function, the black point value x_w is predetermined based on the pixel value that is preferably mapped by the rendering function R(x) to correspond to a photographic black paper density of approximately 2.0. Generally speaking, the aforementioned white point value and dark point value represent first and second predetermined target densities or code values. While these target densities have been described with reference to density measurements of photographic paper, those skilled in the art will recognize that the same approach may be used even if the output device is not photographic paper (a computer monitor, for example.) With the variables x_b and x_ρ defined, the value of the variable α_s1 can be solved for a given value of x_so using expression (10) by the iterative numerical solution. The iterative numerical solution for expressions (9) and (10) includes a process of first estimating an initial value of α_s1, calculating each side of the equation, calculating an error term as the difference, inspecting the error, making an adjustment to the estimate of α_s1, and iterating the procedure until the error term is of an acceptably low magnitude. The iterative solution results are computed for all possible values of α_s1, and stored in a LUT. The same calculations and procedure are used to determine the value of α_h1.

[0087] The variables x_ho and x_so are control variables in the expressions (9) and (10) that, once selected, determine the function shape and slope characteristics for expressions (6) and (7). FIG. 7 shows a graph depicting a family of curves that represent highlight tone scale segments generated with different values of x_ho and a family of curves that represent shadow tone scale segments generated with different values of x_so. Point 500 represents the reference gray point x_ρ. Curve 501 represents a highlight tone scale segment constructed from one highlight component function. Curve 502 represents a shadow tone scale segment constructed from one shadow component function. Line 503 represents the one-to-one input-to-output line. For each of the highlight tone scale segments shown in FIG. 7, the highlight component function graphed has a monotonically decreasing instantaneous slope value. Similarly, for each of the shadow tone scale segments shown in FIG. 7, the component function graphed has a monotonically increasing instantaneous slope value. The variables x_ho and x_so can be selected independently. Thus the shape of the two tone scale segments can be controlled independent from one another.

[0088] The expressions (6) and (7) were derived with the constraint that the slope function (corresponding to the component function), when evaluated at the reference gray point, must be equal to 1.0. In another embodiment, exponential functions are used in similar fashion with a slope constraint imposed. The slope of the highlight component function must be equal to a selected value φ_h and the shadow component function must be equal to a selected value φ_s. For this embodiment, the imposed slope constraint results in a relationship between the variables β_h1 and α_h1 in expression (4) and β_s1 and α_s1 in expression (5) given by expressions (11) and (12) respectively.
β_h1=φ_hα_h1  (11)
β_s1=φ_sα_s1  (12)
The expressions for the highlight component function and the shadow component function are given by expressions (13) and (14) respectively.
ƒ_h1(x)=φ_hα_h1(1−e^{−(x−xρ)/αh1})+x_ρ  (13)
ƒ_s1(x)=φ_sα_s1(1−e^{−(x−xρ)/αs1})+x_ρ  (14)

[0089] The first derivative functions that represent the slope function of the highlight and shadow component functions are given by expressions (15) and (16) respectively.
ƒ_h1′(x)=φ_he^{−(x−xρ)/αh1}  (15)
ƒ_s1′(x)=φ_se^{−(x−xρ)/αs1}  (16)

[0090] If the value of x_ho is greater than the value of x_w, the highlight component function will map a greater range of input pixel values to a lesser range of output pixel values and is therefore considered a compressive function. Conversely, if the value of x_ho is less than the value of x_w, the highlight component function will map a lesser range of pixel values to a greater range of pixel values and is therefore considered an expansive function. Similarly, if the value of x_so is less than the value of x_b, the shadow component function will map a greater range of input pixel values to a lesser range of output pixels value and is therefore considered a compressive function. Conversely, if the value of x_so is greater than the value of x_b, the shadow component function will map a lesser range of pixel values to a greater range of pixel values and is therefore considered an expansive function. Therefore, based on the values of the variables x_ho and x_w, the highlight tone scale segment can be classified as compressive, expansive, or neutral. When the value x_ho is equal to the value of x_w, the highlight tone scale segment is classified as neutral since for this unique condition the highlight tone scale segment assumes the identity mapping function. Based on the values of the variables x_so and x_b, the shadow tone scale segment can be classified as compressive, expansive, or neutral. Similarly, when the value x_so is equal to the value of x_b, the shadow tone scale segment is classified as neutral since for this unique condition the shadow tone scale segment assumes the identity mapping function.

[0091] For compressive highlight component functions, the numerical constant α_h1 is positive. The corresponding slope function of the highlight component function given by expression (15) for positive values of φ_h yields positive slope function values for all x values greater than or equal to the reference gray point x_ρ. The expression for the second derivative function, or the slope function of the slope function of the highlight component function is given by expression (17).
ƒ_h1″(x)=−(φ_h/α_h1)e^{−(x−xρ)/α}_h1  (17)
FIG. 8a shows an example graph of a highlight tone scale segment generated with a φ_h variable set to 1.0, a reference gray point set to 0.0 and a α_h1 variable set to 1.0. As can be seen by inspection of the graph shown in FIG. 8a and by expression (17), positive values of α_h1 and φ_h result in a compressive highlight tone scale segment with a monotonically decreasing slope function that assumes values that are greater than or equal to zero. Curve 512 depicts such a highlight component function used to construct a highlight tone scale segment and curve 513 depicts its corresponding slope function. The reference gray point is indicated by point 511.

[0092] Similarly, for compressive shadow component functions, the numerical constant α_s1 is negative. The corresponding slope function of the shadow component function given by expression (16) for the positive values of φ_s yields positive slope values for all x values less than or equal to the reference gray point x_p. The expression for the second derivative function, or slope function of the slope function of the shadow component function is given by expression (18).
ƒ_s1″(x)=−(φ_s/α_s1)e^{−(x−xρ)/αs1}  (18)
FIG. 8b shows an example graph of a shadow tone scale segment generated with a φ_s variable set to 1.0, a reference gray point set to 0.0 and a α_s1 variable set to 1.0. As can be seen by inspection of the graph shown in FIG. 8b and by expression (18), positive values of α_s1 and φ_s result in a compressive shadow tone scale segment with a monotonically increasing slope function that assumes values that are greater than or equal to zero. Curve 515 depicts such a shadow component function used to construct a shadow tone scale segment and curve 516 depicts its corresponding slope function. The reference gray point is indicated by point 514.

[0093] An example tone scale function 203, shown in FIG. 8c, was constructed from a highlight and shadow tone scale segment, each of which is constructed from a single component function as described above. The equal slope condition, as described above, is not a requirement of the tone scale functions generated by the present invention. In the example tone scale function shown in FIG. 8c, the slopes of the highlight and shadow tone scale segments are not equal when evaluated at the reference gray point. Curve 517 represents the tone scale function 203 and curve 518 its corresponding slope function. The reference gray point is indicated by point 519. As can be seen in the example graph shown in FIG. 8c, the tone scale function 203 has a discontinuity in the slope function at the reference gray point. Tone scale functions having continuous slope function are, in general, desirable. However, experimentation has shown that the discontinuity in the slope function of a tone scale function when applied to digital images relating to natural photographed scenes is not often a problem. Other methods for generating tone scale functions, such as described in commonly-assigned U.S. Pat. No. 6,285,798 have imposed a continuity of slope constraint on the process of construction. The experimentation performed in support of the present invention has found that the continuity of slope constraint can be unnecessarily restrictive for some digital imaging applications. By not imposing the continuity of slope constraint, a greater diversity of useful tone scale functions can be produced. In particular, tone scale functions that are constructed in a manner responsive to the pixels of the source digital image when applied to the source digital image can achieve a greater level of overall contrast enhancement.

[0094] In another embodiment, the highlight tone scale segment is constructed from a compressive highlight component function. Recall that for compressive highlight component functions x_ho is greater than x_w. For this embodiment, the expression given by (6) is combined with a linear function which relaxes the function's slope condition at the reference gray point. The expression for the highlight component function is given by (19)
ƒ_h1(x)=(1−φ_HC)α_h1(1−e^{−(x−xρ)/αh1})+φ_HCγ_HC(x−x_ρ)+x_ρ  (19)
where the variable γ_HC represents the average slope for the function over the interval from x_ρ to x_ho and is given by expression (20).
γ_HC=(x_w−x_ρ)/(x_ho−x_ρ)  (20)
The variable φ_HC determines the contribution of the linear function to the highlight component function. The variable φ_HC can be selected to affect a change in the shape of the highlight tone scale segments that uses expression (19) as a highlight component function. If φ_HC is set to 0.0, expression (19) reverts to expression (6). If φ_HC is set to 1.0, the expression (19) assumes a linear function given by expression (21).
ƒ_h1(x)=γ_HC(x−x_ρ)+x_ρ  (12)
Thus the variable φ_HC is a control parameter that can be used to select the degree to which the highlight component function behaves as a pure exponential function. Similarly, the expression for the shadow component function is given by (22)
ƒ_s1(x)=(1−φ_SC)α_s1(1−e^{−(x−xρ)/αs1})+φ_SCγ_SC(x−x_ρ)+x_ρ  (22)
where the variable γ_SC represents the average slope for the function over the interval from x_so to x_ρ and is given by expression (23).
γ_SC=(x_b−x_ρ)/(x_so−x_ρ)  (23)
The variable φ_SC can be selected to change the shape of the shadow tone scale segments that use expression (22) as a shadow component function. If φ_SC is set to 0.0, expression (22) reverts to expression (7). If φ_SC is set to 1.0, expression (22) assumes a linear function given by expression (24).
ƒ_s1(x)=γ_SC(x−x_ρ)+x_ρ  (24)
Thus the variable φ_SC is a control parameter that can be used to select the degree to which the shadow component function behaves as a pure exponential function.

[0095] Changing the φ_HC and φ_SC variables can have a significant impact on the appearance of the processed digital images. Setting the φ_HC and φ_SC variables toward 0.0 results in processed digital images that have a more traditional photographic high contrast appearance. Conversely, setting the φ_HC and φ_SC variables toward 1.0 results in processed digital images that have a more professional photographic low contrast appearance more appropriate for portraiture. FIG. 9 shows a graph depicting a family of curves that represent highlight tone scale segments generated with expression (19) using different values of x_ho and a family of curves that represent shadow tone scale segments generated with expression (22) using different values of x_so. Point 521 represents the reference gray point x_ρ. Curve 522 represents a highlight tone scale segment constructed from one highlight component function. Curve 523 represents a shadow tone scale segment constructed from one shadow component function. Line 524 represents the identity mapping one-to-one input pixel value-to-output pixel value line. The tone scale function 203 can be constructed from any of the highlight tone scale segments depicted in FIG. 9 or any of the shadow tone scale segments since the variables x_ho and x_so can be selected independently. It should also be noted that if the variables φ_HC and φ_SC are not set to 0.0, the resultant tone scale function will, in general, not have a continuous slope function at the reference gray point.

[0096] A highlight component function constructed with expressions (4), (6), or (13), for the case in which x_ho is less than x_w, will result in a function that has a monotonically increasing instantaneous slope. An example of such a function is depicted as curve 530 in FIG. 10a. While the function indicated by curve 530 satisfies the mapping of input pixel value x_ho to output pixel value x_w, a tone scale function based on such a function can produce some unnatural looking images. This is mainly due to the fact that the slope of the highlight component function depicted by curve 530 has a slope that is monotonically increasing, not monotonically decreasing, for the input pixel range near the reference gray point. However, a highlight component function can be constructed using the expressions (4), (6), or (13), reflected about the line indicated as line 531 given by (25) for input pixel values greater than or equal to the reference gray point (indicated by point 533).
y(x)=(x_w−x_ρ)/(x_ho−x_ρ)(x−x_ρ)+x_ρ  (25)
The function produced by the reflection process, as indicated by curve 532 in FIG. 10a, is a monotonically increasing function with a monotonically decreasing instantaneous slope. The functional form of the expansive highlight component function is calculated by the following steps. First the expression (4), (6), or (13) is solved using the constraint x_ho=x_w to determine the numerical constant α_h1. In the second step, a first rotation transform is applied to a coordinate pair (x,ƒ(x)) resulting in a transformed coordinate pair (u,v) as given by (26)
u=x cos(θ)+ƒ(x)sin(θ)  (26)
v=−x sin(θ)−ƒ(x)cos(θ)
where the angle θ is given by (27).
θ=tan⁻¹((x_w−x_ρ)/(x_ho−x_ρ))  (27)
The first rotation transform is designed to transform the line described by expression (24) into the x-axis. In the third step the v coordinate is reflected about the new x-axis by taking the negative of the value v coordinate. In the fourth step, a reverse rotation transform is applied to the coordinate pair (u, −v) for the coordinate pair (u′,v′) as given by (28).
u′=u cos(θ)+v sin(θ)  (28)
v′=x sin(θ)−v cos(θ)
In the fifth step, the coordinate pair (u′,v′) defines a highlight component function g(u) and is evaluated for the range of input pixel values.

[0097] Referring to FIG. 10b, an expansive shadow component function can be constructed in similar manner as described above for the case in which x_so>x_b. The shadow component function can be constructed using the function described by (5), (7), or (14), reflected about the line indicated as line 535 given by (29) for input pixel values less than or equal to the reference gray point (indicated by point 534) using the constraint x_so=x_b to determine the numerical constant α_s1.
y(x)=x_b−x_ρ)/(x_so−x_ρ)(x−x_ρ)+x_ρ  (29)
The curve indicated by 536 depicted in FIG. 10b shows a function using (5), (7), or (14) for a positive numerical constant α_s1. The corresponding expansive shadow component function produced via the reflection processing steps is indicated by curve 537.

[0098] An example graph of a tone scale function constructed from an expansive highlight tone scale segment and an expansive shadow tone scale segment constructed with the component functions described above is shown in FIG. 10c. Since both the highlight and shadow tone scale segments are expansive, the tone scale function, indicated by curve 539, is classified as an expansive function over the range of input pixel values from x_so to x_ho. The combined linear functions given by expressions (25) and (29) are shown as line 540 with the reference gray point as indicated by point 538. The tone scale function depicted in FIG. 8c as curve 517 is classified as a compressive function over the range of input pixel values from x_so to x_ho since both the highlight and shadow tone scale segments used in its construction are compressive component functions. The reference gray point is indicated by point 538.

[0099] It is possible to construct a tone scale function from an expansive shadow component function and a compressive highlight component function or from a compressive shadow component function and an expansive highlight component function since the shape of the two tone scale segments are independently controllable. Such a tone scale function is referred to herein as an eclectic function since the two segments are of different shape classification. While is would be possible to have a tone scale applicator module specific to the eclectic function classification category, the present invention uses the compressive function applicator 242 to apply the tone scale function 203 for eclectic functions. Thus, the eclectic functions are reclassified as compressive functions. For tone scale functions constructed from a highlight and shadow tone scale segment, the present invention classifies the tone scale function 203 as compressive if either tone scale segment individually has been classified as compressive. Otherwise the tone scale function 203 is classified as expansive. It should be noted that other classification rule sets can be used. For example, the eclectic tone scale functions can be classified as expansive tone scale functions.

[0100] In a preferred embodiment, the expansive highlight component function ƒ_h1(x) is constructed using the expression (4) subject the constraints given by (30) and (31).
ƒ_h1(x_ho′)=x_w  (30)
ƒ_h1′(x_w)=1.0  (31)
where the variable x_ho′ is given by the expression (32)
ƒ_h1(x_ho′)=(1.0−η_H)(x_w−x_ho)+x_ho  (32)
and the variable η_H represents a control parameter that can be used to select the shape of the function. With these two constraints placed on the function ƒ_h1(x), the highlight component function achieves the goal of mapping the prescribed input pixel value x_ho′ to the prescribed output pixel value x_w. The average slope of the function γ_HE over the interval from x_ρ to x_ho′ is given by the expression (33)
γ_HE=(x_w−x_ρ)/(x_ho′−x_ρ)  (33)
which is greater than 1.0 since x_w is greater than x_ho′. In a similar manner as described above, the variables α_h1 and β_h1 used in the expression (4) are solved by iterative numerical approximation and stored in a LUT for later recall. The variable η_H is preferably set to 0.5. As a further refinement, the highlight component function is combined with a linear function using a control parameter φ_HE. The final expression for the expansive highlight component function is given by (34).
ƒ_h1(x)=(1−φ_HE)α_h1(1−e^{−(x−xρ)/αh1})+φ_HEγ_HE+x_ρ  (34)

[0101] Similarly, the expansive shadow component function ƒ_s1(x) is constructed using the expression (5) subject the constraints given by (35) and (36).
ƒ_s1(x_so′)=x_b  (35)
ƒ_s1′(x_b)=1.0  (36)
where the variable x_so′ is given by the expression (37)
ƒ_s1(x_so′)=(1.0−η_S)(x_b−x_so)+x_so  (37)
and the variable η_S represents a control parameter that can be used to select the shape of the function. With these two constraints placed on the functions ƒ_s1(x), the shadow component function achieves the goal of mapping the prescribed input pixel value x_so′ to the prescribed output pixel value x_b. The average slope of the function γ_SE over the interval from x_so′ to x_ρ is given by the expression (38)
γ_SE=(x_ρ−x_b)/(x_ρ−x_so′)  (38)
which is greater than 1.0 since x_b is less than x_so′. In a similar manner as described above, the variables α_s1 and β_s1 used in the expression (5) are solved by iterative numerical approximation and stored in a look-up-table (LUT) for later recall. The variable η_S is preferably set to 0.5. As a further refinement, the shadow component function is combined with a linear function using a control variable φ_SE. The final expression for the expansive shadow component function is given by (39).
ƒ_s1(x)=(1−φ_SE)α_s1(1−e^{−(x−xρ)/αs1})+φ_SEγ_SE+x_ρ  (39)
FIG. 11 depicts a family of highlight and shadow tone scale segments each generated from a single expansive component function using expressions (34) and (39) respectively. Point 541 represents the reference gray point x_ρ. Line 542 represents the identity mapping one-to-one input pixel value-to-output pixel value line. Curve 543 represents an example a highlight tone scale segment using expression (34). Curve 544 represents an example shadow tone scale segment using expression (39).

[0102] An important feature of the tone scale segments constructed with the two-segment method is the gradual transition in function shape corresponding to a transition from compressive to expansive type functions. FIG. 12a depicts a family of highlight tone scale segments constructed from a single highlight component function using expression (19) when the component function is compressive and expression (34) when the component function is expansive. Point 549 indicates the reference gray point. Curve 550 indicates a highly compressive highlight component function while curve 551 indicates a mildly highlight component function. The higher the degree of compression, the greater the curvature of the function. When the highlight component function is neither compressive nor expansive, the function assumes the identity mapping one-to-one input-to-output line indicated by line 552. Curve 553 indicates a mildly expansive highlight component function. Curve 554 indicates a highly expansive highlight component function. The higher the degree of expansion, the greater the curvature of the function. Thus for mildly compressive and mildly expansive highlight component functions, the shape of the function is closer to a straight line. For the compressive highlight component functions used to construct the highlight component segments as shown in FIG. 12a a value of 0.0 was used for the variable φ_HC. For the expansive highlight component functions used to construct the highlight component segments shown in FIG. 12a a value of 0.0 was used for the variable φ_HE. It should be noted that the value of the φ_HC variable used in expression (19) can be selected independent of the value of the φ_HE variable used in expression (34).

[0103] The shape of the shadow tone scale segments also have a graceful transition between compressive and expansive function types. FIG. 12a also depicts a family of shadow tone scale segments constructed from a single shadow component function using expression (22) when the component function is compressive and expression (39) when the component function is expansive. Curve 555 indicates a highly compressive shadow component function while curve 556 indicates a mildly shadow component function. The higher the degree of compression, the greater the curvature of the function. When the shadow component function is neither compressive nor expansive, the function assumes the identity mapping one-to-one input-to-output line indicated by line 557. Curve 558 indicates a mildly expansive shadow component function. Curve 559 indicates a highly expansive shadow component function. The higher the degree of expansion, the greater the curvature of the function. Thus for mildly compressive and mildly expansive shadow component functions, the shape of the function is closer to a straight line. For the compressive shadow component functions used to construct the shadow component segments as shown in FIG. 12a a value of 0.0 was used for the variable φ_SC. For the expansive shadow component functions used to construct the shadow component segments shown in FIG. 12a a value of 0.0 was used for the variable φ_SE. It should be noted that the value of the φ_SC variable used in expression (22) can be selected independent of the value of the φ_SE variable used in expression (39).

[0104] FIG. 12b depicts a similar family of highlight and shadow tone scale segments constructed using expressions (19), (34), (22) and (39) with the values of the variables φ_HC, φ_HE, φ_SC, and φ_SE all set to a value of 0.5. As can be seen from the curves depicted in FIG. 12b, the tone scale segments make a gradual transition in function shape for a corresponding transition from compressive to expansive type functions.

[0105] In another construction method for a compressive highlight component function, the variable φ_h is made a function of the degree of compression, i.e. the ratio of (x_w−x_ρ) to (x_ho−x_ρ). The expression for the highlight component function ƒ_h1(x) is given by (13) where the slope variable φ_h in expression (13) is given by (40)
φ_h=1.0−η_h(1.0−(x_w/x_ho))  (40)
and the variable η_h controls the shape of the highlight component function. The variable η_h can be selected. When the variable η_h is set to 1.0, the highlight component function assumes the equation of a line given as expression (21). When the variable η_h is set to 0.0, the highlight component function assumes the equation of the exponential function given as expression (13). Similarly, for a compressive shadow component function, the variable φ_s is made a function of the degree of compression, i.e. the ratio of (x_ρ−x_b) to (x_ρ−x_so). The expression for the shadow component function ƒ_s1(x) is given by (14) where the slope variable φ_s in expression (14) is given by (41)
φ_s=1.0−η_s(1.0−(x_b/x_so))  (41)
and the variable η_s controls the shape of the shadow component function. When the variable η_s is set to 1.0, the shadow component function assumes the equation of a line given as expression (24). When the variable η_s is set to 0.0, the shadow component function assumes the equation of the exponential function given as expression (14).

[0106] Referring to FIG. 4, the pixels of the source digital image, i.e. the input digital image to the tone scale module 330, can be used to determine the variables x_ho and x_so and thus determine the shape of the tone scale function. The analysis image generator 250 receives the source digital image and produces an analysis digital image 201 by applying a low-pass spatial filter and performing a sampling operation. The result is a lower spatial resolution version of the source digital image that has fewer pixels representing the same image content. Typical spatial resolutions for the analysis digital image 201 is approximately 64 by 96 pixels. The analysis digital image is also converted to be in a luminance-chrominance representation using expression (2).

[0107] The tone scale function generator 230 calculates the tone scale function 203 by analyzing the pixels of the analysis digital image 201. A pixel histogram function, i.e. a frequency of occurrence function, is calculated from the pixels of the luminance digital image channel of the analysis digital image 201. A cumulative histogram function is calculated from the pixel histogram function by integrating the values of the pixel histogram as a function of pixel value. FIG. 14 shows a graph of an example histogram function indicated by curve 601 and its corresponding cumulative histogram function indicated by curve 602. The cumulative histogram function is scaled between 0.0 and 1.0 and relates to the percentage area of the digital image with 1.0 corresponding to 100 percent of image area. The ordinate value of the cumulative histogram function relates to a given percentage image area Z. The corresponding abscissa value relates to the pixel value P for which the percentage of pixels in the image that have values less than P is given by Z. The 99.0% cumulative histogram function value is used to determine the value of the highlight point, variable x_ho. The 1.0% cumulative histogram function value is used to determine the value of the shadow point, variable x_so. The values for the variables x_ho and x_so are indicated as points 603 and 604 respectively on the graph shown in FIG. 14.

[0108] Referring to FIG. 3 and FIG. 4, the tone scale function generator 230 can accept user input selections 231 in the generation of the tone scale function 203. For example a user of the system shown in FIG. 1 can view possible selections and/or control parameters on the monitor device 50 and indicate selections using the input control device 60 such as a keyboard or mouse pointing device. When used in this manual user mode, a highlight control parameter and a shadow control parameter are used to change the shape of the component functions that are used to generate the highlight tone scale segment and shadow tone scale segment respectively.

[0109] The present invention is preferably practiced in an image processing system including a source of digital images, such as a scanner, a computer programmed to process digital images, and an output device such as a thermal or ink-jet printer. The method of the present invention may be sold as a computer program product including a computer readable storage medium bearing computer code for implementing the steps of the invention.

[0110] Computer readable storage medium may include, for example; magnetic storage media such as a magnetic disc (e.g. a floppy disc) or magnetic tape; optical storage media such as optical disc or optical tape; bar code; solid state electronic storage devices such as random access memory (RAM) or read only memory (ROM); or any other physical device or medium employed to store a computer program.

[0111] The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

• [0112] 10 image capture device
• [0113] 10a image capture device
• [0114] 10b image capture device
• [0115] 10c image capture device
• [0116] 20 digital image processor
• [0117] 30a image output device
• [0118] 30b image output device
• [0119] 40 general control processor
• [0120] 50 monitor device
• [0121] 60 input control device
• [0122] 70 computer memory device
• [0123] 101 original digital image
• [0124] 102 source digital image
• [0125] 103 rendered digital image
• [0126] 104 enhanced digital image
• [0127] 107 luminance digital image
• [0128] 109 chrominance digital image
• [0129] 113 enhanced luminance digital image
• [0130] 201 analysis digital image
• [0131] 203 tone scale function
• [0132] 210 LCC conversion module
• [0133] 220 RGB conversion module
• [0134] 230 tone scale function generator
• [0135] 231 user input selections
• [0136] 240 tone scale function applicator
• [0137] 241 tone scale function classifier
• [0138] 242 compressive function applicator
• [0139] 244 expansive function applicator
• [0140] 250 analysis image generator

• [0141] 310 RLSE conversion module
• [0142] 320 scene balance module
• [0143] 330 tone scale module
• [0144] 340 rendering module
• [0145] 401 input digital image
• [0146] 402 texture digital image
• [0147] 403 pedestal digital image
• [0148] 407 tone scale adjusted digital image
• [0149] 409 output digital image
• [0150] 410 pedestal generation module
• [0151] 420 difference module
• [0152] 430 tone scale modifier
• [0153] 432 chrominance modifier
• [0154] 440 addition module
• [0155] 500 point
• [0156] 501 curve
• [0157] 502 curve
• [0158] 503 line
• [0159] 511 point
• [0160] 512 curve
• [0161] 513 curve
• [0162] 514 point
• [0163] 515 curve
• [0164] 516 curve
• [0165] 517 curve
• [0166] 518 curve
• [0167] 519 point
• [0168] 521 point
• [0169] 522 curve

• [0170] 523 curve
• [0171] 524 line
• [0172] 530 curve
• [0173] 531 line
• [0174] 532 curve
• [0175] 533 point
• [0176] 534 point
• [0177] 535 line
• [0178] 536 curve
• [0179] 537 curve
• [0180] 538 point
• [0181] 539 curve
• [0182] 540 line
• [0183] 541 point
• [0184] 542 line
• [0185] 543 curve
• [0186] 544 curve
• [0187] 549 point
• [0188] 550 curve
• [0189] 551 curve
• [0190] 552 line
• [0191] 553 curve
• [0192] 554 curve
• [0193] 555 curve
• [0194] 556 curve
• [0195] 557 line
• [0196] 558 curve
• [0197] 559 curve

• [0198] 590 curve
• [0199] 591 point
• [0200] 592 point
• [0201] 593 point
• [0202] 595 point
• [0203] 596 point
• [0204] 597 point
• [0205] 598 curve
• [0206] 601 curve
• [0207] 602 curve
• [0208] 603 point
• [0209] 604 point